1. Field of the Invention
The present invention relates to a solid electrolytic capacitor and a method for preparing the same.
2. Description of the Prior Art
Heretofore, a solid electrolytic capacitor has comprised an anode, a dielectric layer, an electrolyte layer, and a cathode. Generally, a solid electrolytic capacitor has had a structure comprising an anode made of a metal exhibiting valve action (valve metal), an oxidized layer as a dielectric layer formed over the surface of the anode, an electrolyte layer as a semiconductor layer formed on the dielectric layer, and a cathode (made of graphite or the like) formed on the electrolyte layer.
In this connection, the valve metal means a metal capable of forming an oxidized layer whose thickness can be controlled by anodic oxidation. Specifically, valve metal includes niobium. (Nb), aluminum (Al), tantalum (Ta), titanium (Ti), hafnium (Hf) and zirconium (Zr). Actually, however, aluminum and tantalum are mainly used.
With respect to aluminum (Al), a foil is generally used as the anode, and with respect to Ta, a porous body prepared by sintering a Ta-based powder is used as the anode.
Of those solid electrolytic capacitors, a solid electrolytic capacitor having a porous sintered body type is particularly adaptable to miniaturization and capable of being adapted to have a high capacity, and hence there is strong demand therefor as a part which meets needs of miniaturization of a cellular phone, information terminal equipment or the like.
In the following, a structure and a preparation method of a conventional Ta solid electrolytic capacitor will be described with reference to the drawings.
FIG. 4 is a sectional view showing a structure of a conventional Ta solid electrolytic capacitor.
As shown in FIG. 4, the conventional solid electrolytic capacitor 1 using Ta comprises an anode body 11 which is provided with an element lead wire 11a implanted therein and which Is formed by sintering a Ta-based mixed powder, a dielectric layer 12 formed over the surface of the anode body 11, an electrically conductive polymer layer as an electrolyte layer 13 which is formed on the surface of the dielectric layer 12, a graphite paste layer 14 as a cathode body which is formed on the electrolyte layer 13 as a semiconductor layer, and a silver (Ag)-containing paste layer 15 formed on the cathode body 14.
To the element lead wire 11a of the anode body 11 and the silver (Ag)-containing paste layer 15, lead frames 52 are connected, respectively. The resultant is sheathed with a resin by molding with end portions of the lead frames 52 out.
In the next place, a method for preparing a conventional Ta solid electrolytic capacitor will be described with reference to FIG. 5.
FIG. 5 is a flow chart showing a method for preparing a conventional solid electrolytic capacitor.
(1) Formation of Ta Porous Body (S 1)
(i) Preparation of Ta-based Powder
To improve press-moldability, a binder is added to a Ta powder, and the addition is followed by mixing.
(ii) Press Molding and Sintering
An element lead wire for an anode is partially inserted in the Ta-based powder, and the resultant was press-molded into a cylindrical or parallelepipedonal shape
Then, the press-molded product is sintered by heating at a temperature of 1,300degreeC. to 2,000degreeC. under high vacuum (10xe2x88x924 Pa or lower pressure) to form a Ta porous body, i.e., an anode body.
(2) Formation of Dielectric Layer (S 2)
Chemical Conversion Treatment (S 2a)
The Ta porous body as an anode was soaked in an electrolytic aqueous solution such as a phosphoric acid aqueous solution together with a counter electrode, and a chemical conversion voltage (formation voltage) is applied to thereby form an oxidized Ta layer as a dielectric layer over the surface of the Ta porous body. (anodic oxidation method)
The thickness of the dielectric layer (oxidized Ta layer) is dependent upon the chemical conversion voltage (Vf: formation voltage) when an electric current is fixed, and characteristics as a capacitor are in turn dependent upon the thickness of the oxidized Ta layer. As the electrolytic solution, there may be used an aqueous solution of phosphoric acid of which concentration is adjusted to 0.6%, or the like.
(3) Formation of Electrolyte Layer (S 3)
On the oxidized layer formed over the Ta porous body in the preceding step, a solid electrolyte layer is formed. (S 3a)
As the solid electrolyte, there may be used manganese dioxide, or an electrically conductive polymer obtained by polymerizing a monomeric material such as pyrrole, thiophene or a derivative thereof.
For example, when a pyrrole polymer is used as the solid electrolyte, a solid electrolyte layer is formed on the dielectric layer formed over the surface of the anode body by effecting chemical polymerization or electrolytic polymerization using a pyrrole monomer solution and a solution of an oxidizing agent such as iron trichloride, as disclosed in Japanese Unexamined Patent Publication No.2001-160318 by Fukunaga et al.
For forming the electrically conductive polymer, a process may be employed which comprises preliminarily applying an oxidizing agent to the surface of the dielectric layer, and then soaking the resultant in a monomer solution to effect polymerization reaction, as disclosed in Japanese Unexamined Patent Publication No.2000-216061 by one of the present inventors.
(4) Re-Treatment for Chemical Conversion (S 4)
In the step of forming the solid electrolyte layer, the dielectric layer is likely to be damaged. To mend the damaged portions of the dielectric layer, the anode body with the sequentially formed dielectric and solid electrolyte layers is subjected again to the chemical conversion treatment.
(5) Formation of Cathode Body (S 5)
Formation of Graphite Paste Layer (S 5a), and Formation of Silver (Ag)-Containing Paste Layer (S 6)
A graphite layer as a cathode layer is formed on the solid electrolyte layer, and a silver (Ag)-containing paste layer is formed thereon. With respect to the formation of the graphite layer, a method disclosed in Japanese Unexamined Patent Publication NC. 1999-297574 by one of the present inventors may be employed.
(6) Connection of Lead Frames (S 7), and Sheathing by Molding (S 8)
Then, a lead frame for the anode is connected to the element lead wire of the anode body by spot welding, and a lead frame for the cathode is connected to the silver (Ag)-containing paste layer with an electrically conductive adhesive.
Finally, the resulting capacitor element is sheathed with a resin by molding with end portions of the lead frames out to complete a Ta solid electrolytic capacitor having a structure as shown in FIG. 4.
However, the Ta solid electrolytic capacitor prepared through the above-described steps has problems which affect basic characteristics thereof, i.e., formation of so-called defective portions and lowering of electrical conductivity of the electrolyte layer.
In the first place, description will be given with respect to the defective portions of the dielectric layer.
The defective portions of the dielectric layer mean portions where the dielectric layer has insufficient thicknesses, and specifically, mean locally concave areas where the dielectric layer has smaller thicknesses such as cracks or concave areas having thicknesses smaller than the intended thickness of the dielectric layer such as portions which have undergone exfoliation of the dielectric layer.
Causes of the formation of defective areas include (A) inclusion of impurities in Ta, (B) irregularity of current in the chemical conversion step and (C) external mechanical stress, and it is believed that any of these causes cannot completely be prevented from occurring in the existing methods for preparing a solid electrolytic capacitor.
Since thicknesses of the dielectric layer in the defective portions are smaller than that in the other area, local intensifications of field strength are likely to occur in such defective portions. Locally elevated temperatures caused by the local intensifications of field strength rise to crystallizations of the dielectric layer to result in dielectric breakdowns of the dielectric layer. (The term xe2x80x9clocal intensifications of field strengthxe2x80x9d used herein means a phenomenon that electric field strength is locally intensified.)
Such dielectric breakdowns make one of the causes of increase in leakage current (hereinafter referred to as LC).
In the conventional technique, the step of re-treatment for chemical conversion is employed as means for repairing the defective portions of the dielectric layer. However, the step of re-treatment for chemical conversion is insufficient as means for repairing the dielectric layer. For example, the step does not rake measures to solve the above-mentioned cause (A). This is because if a metal which does not exhibit valve action, for example, a metal more noble than tantalum is present on the surface of the anode body as an impurity, no substantial dielectric layer is formed over the metal present on the surface of the anode body.
In the next place, description will be given with respect to the lowering of electrical conductivity of the electrolyte layer.
The electrolyte layer of the conventional solid electrolytic capacitor prepared by the above-described conventional preparation method comprises an electrically conductive polymer in the form of deposited clusters. Contact resistances at contacts between the clusters and contacts between the clusters and the cathode body cause increase of equivalent series resistance (hereinafter referred to as ESR). A capacitor having high ESR has a drawback that its characteristics in a high frequency range are poor.
To cope therewith, electrically conductive particles are incorporated at an appropriate density so as to diminish the contacts mentioned above to reduce contact resistances in the electrolyte layer. Hence, and the electrolyte layer is thereby improved in its function as a semiconductor layer to obtain a solid electrolytic capacitor having low ESR, as disclosed in JP-P-2001-307958A and JP-P-2002-15956A by Mitsui et al., JP-P-2000-133551A by Hoshino et al., and JP-P-2000-191906A by Yoshilawa et al.
The electrically conductive particles are only required to be present between the clusters and in the interface between the clusters and the cathode body. The electrically conductive particles may be present in the electrically conductive polymer clusters.
The electrically conductive particles preferably have sizes in a range of about 1 xcexcm to about 1000 xcexcm which enables effectively improving electrical conductivity of the electrolyte layer.
The electrically conductive particles may be made of the same material as constitutes the cathode body formed on the electrolyte layer. The electrically conductive particles may be made of one selected from the electrically conductive materials such as a powder of SnO2 or ZnO, inorganic particles of TiO2, BaSO4 or the like which have the above-listed powder thereon and thereby rendered electrically conductive, an electrically conductive carbonaceous filler such as carbon black, particulate graphite or fine carbon fiber, and a powder of an electrically conductive obtained by polymerizing a monomeric material such as pyrrole, thiophene or a derivative thereof.
In the above-described prior art also, the electrically conductive particles are placed between the cathode body and dielectric layer with a view to lowering ESR.
However, there is an upper limit with respect to the density of the electrically conductive particles which allows the electrolyte layer to effectively function as a semiconductor layer. In other words, incorporation of the electrically conductive particles in an excess amount causes deterioration of basic characteristics of the capacitor, in particular, increase of LC.
Accordingly, the electrically conductive particles are generally incorporated in a controlled amount within a range predetermined on the supposition that the electrically conductive particles are uniformly dispersed.
In reality, however, even if the electrically conductive particles are incorporated in an amount estimated to be effective, increase of LC can be caused.
This is because the electrically conductive particles can be distributed non-uniformly due to thermal expansion, heat contraction or the like caused in the steps of the preparation of the solid electrolytic capacitor, in particular, the step of molding a resin or the step of welding to form areas where the electrically conductive particles are distributed locally at high densities on the surface of the dielectric layer. In such areas, local intensifications of field strength are likely to occur to cause increase of LC.
Further, even if the electrically conductive particles are incorporated in an appropriate amount and therefore the electrically conductive particles are distributed at moderate densities, increase of LC is able to occur. In a case where the electrically conductive particles are present between the defective portions of the dielectric layer and the electrically conductive polymer, local intensifications of field strength occurs to cause increase of LC consequently.
In the conventional techniques, the electrically conductive particles are incorporated in the electrolyte layer focusing attention only on lowering ESR, and as opposed to the present invention, no attention is paid to the disadvantage attendant on the incorporation of the electrically conductive particles. Accordingly, studies have not been made from the viewpoint to realize compatibility between low ESR and low LC.
It is the primary object of the present invention to provide a solid electrolytic capacitor which is capable of solving the above-mentioned problems and realizing compatibility of low LC with low ESR, and a method for preparing the same.
Specifically, the object of the present invention is to provide a solid electrolytic capacitor which is not susceptible to dielectric breakdown of a dielectric layer, and a method for preparing the same.
According to an embodiment of the present invention, there is provided a solid electrolytic capacitor comprising:
an anode body;
a dielectric layer formed over the surface of the anode body;
non-conductive particles placed on at least a part or the dielectric layer;
an electrolyte layer formed on the dielectric layer with the non-conductive particles placed thereon, said electrolyte layer including an electrically conductive polymer and electrically conductive particles; and
a cathode body formed on the electrolyte layer.
In the solid electrolytic capacitor having the constitution, dielectric breakdowns of the dielectric layer are prevented to a large extent to diminish possibility of increase of LC, and compatibility of low LC with low ESR is thereby realized.
According to another embodiment of the present invention, there is provided a method for preparing a solid electrolytic capacitor, said method comprising steps of:
(A) forming a dielectric layer over a surface of an anode body;
(B) forming an electrolyte layer including electrically particles on the dielectric layer;
(C) applying a colloidal dispersion containing non-conductive colloidal particles to the product resulting from the step (B), followed by drying the resultant to place the non-conductive particles in at least a part of the interface between the dielectric layer and the electrolyte layer; and
(D) forming a cathode body on the electrolyte layer.
By employing the method, the non-conductive particles are effectively placed in the interface between the dielectric layer and the electrolyte layer to realize low LC, thereby contributing to improvement of characteristics of the capacitor. Accordingly, proportion defective in the preparation is lowered to lead to improvement in yield of the solid electrolytic capacitor.